With the increasing costs of fuel it has become important in the field of combustion to carry out combustion with decreased amounts of fuel. It has long been known that one way for reducing fuel consumption is to carry out the combustion using oxygen or oxygen-enriched air as the oxidant as this reduces the amount of nitrogen passing through the combustion reaction which absorbs heat generated by the combustion reaction. Thus more heat is available for the intended purpose for any given amount of fuel.
One problem with the use of oxygen or oxygen-enriched air as the oxidant in a combustion reaction is that the higher temperatures resulting from such reaction kinetically favor the formation of nitrogen oxides (NO.sub.x), which are known pollutants. Recent work in combustion technology has addressed this issue and has resulted in advances which enable the use of oxygen or oxygen-enriched air in combustion processes without generating inordinate levels of NO.sub.x. Several such recent advances in combustion technology are disclosed and claimed in U.S. Pat. No. 4,378,205--Anderson, U.S. Pat. No. 4,541,796--Anderson, U.S. Pat. No. 4,907,961--Anderson, and U.S. Pat. No. 5,100,313--Anderson et al.
Many of these recent advances which enable the use of oxygen or oxygen-enriched air in combustion processes while avoiding the generation of large quantities of NO.sub.x require that the oxidant be provided into the combustion zone at a high velocity. Herein lies a problem because high velocity injection requires a high oxidant pressure and often the oxidant is not available from its source at the requisite high pressure. In these situations the oxidant generally is compressed to the requisite high pressure by use of a compressor. This is undesirable because the use of a compressor increases both the capital and operating costs of the combustion system and also because compressing oxidant, especially when the oxidant is technically pure oxygen, introduces a degree of risk to the system.
It is desirable therefore to have a method whereby the velocity of oxidant injected into a combustion zone may be increased over what would otherwise be possible given the source pressure of the oxidant, while reducing or eliminating the need for compressing the oxidant by passing it through a compressor.
It is known that one way for increasing the velocity of oxidant injected into a combustion zone is to inject the oxidant into the combustion zone through a mechanical nozzle. A mechanical nozzle is an orifice having a reduced diameter over that of the oxidant supply conduit so that as the oxidant passes through the orifice into the combustion zone its velocity is increased. However, this has only limited benefit because a relatively high pressure is still required to effectively pass the oxidant through the narrow orifice. Another important limitation of a mechanical nozzle system is that for a given oxidant flow rate the velocity of the oxidant injected into the combustion zone cannot be changed without also changing the nozzle or using a very complicated adjustable nozzle. Changing nozzles is cumbersome and time consuming as well as being dangerous, and adjustable nozzles are expensive and also prone to breakdown when exposed to the harsh environment of a combustion zone.
Accordingly, it is an object of this invention to provide a combustion method wherein the velocity of oxidant injected into a combustion zone may be increased over what would otherwise be possible given the source pressure of the oxidant, while reducing or eliminating the need for compressing the oxidant by passing it through a compressor.
It is another object of this invention to provide a combustion method wherein the velocity of oxidant injected into a combustion zone, for any given supply pressure, may be controlled or varied without the need for changing injection nozzles or using an adjustable nozzle.